Change PluginCommand API to be more like Command (#12279)

# Description

This is something that was discussed in the core team meeting last
Wednesday. @ayax79 is building `nu-plugin-polars` with all of the
dataframe commands into a plugin, and there are a lot of them, so it
would help to make the API more similar. At the same time, I think the
`Command` API is just better anyway. I don't think the difference is
justified, and the types for core commands have the benefit of requiring
less `.into()` because they often don't own their data

- Broke `signature()` up into `name()`, `usage()`, `extra_usage()`,
`search_terms()`, `examples()`
- `signature()` returns `nu_protocol::Signature`
- `examples()` returns `Vec<nu_protocol::Example>`
- `PluginSignature` and `PluginExample` no longer need to be used by
plugin developers

# User-Facing Changes
Breaking API for plugins yet again 😄

crates/nu_plugin_custom_values/src/generate2.rs

@@ -1,35 +1,43 @@
 use crate::{second_custom_value::SecondCustomValue, CustomValuePlugin};
 use nu_plugin::{EngineInterface, EvaluatedCall, SimplePluginCommand};
-use nu_protocol::{
-    Category, LabeledError, PluginExample, PluginSignature, Span, SyntaxShape, Value,
-};
+use nu_protocol::{Category, Example, LabeledError, Signature, Span, SyntaxShape, Value};
 
 pub struct Generate2;
 
 impl SimplePluginCommand for Generate2 {
     type Plugin = CustomValuePlugin;
 
-    fn signature(&self) -> PluginSignature {
-        PluginSignature::build("custom-value generate2")
-            .usage("PluginSignature for a plugin that generates a different custom value")
+    fn name(&self) -> &str {
+        "custom-value generate2"
+    }
+
+    fn usage(&self) -> &str {
+        "PluginSignature for a plugin that generates a different custom value"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
             .optional(
                 "closure",
                 SyntaxShape::Closure(Some(vec![SyntaxShape::Any])),
                 "An optional closure to pass the custom value to",
             )
             .category(Category::Experimental)
-            .plugin_examples(vec![
-                PluginExample {
-                    example: "custom-value generate2".into(),
-                    description: "Generate a new SecondCustomValue".into(),
-                    result: Some(SecondCustomValue::new("xyz").into_value(Span::test_data())),
-                },
-                PluginExample {
-                    example: "custom-value generate2 { print }".into(),
-                    description: "Generate a new SecondCustomValue and pass it to a closure".into(),
-                    result: None,
-                },
-            ])
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                example: "custom-value generate2",
+                description: "Generate a new SecondCustomValue",
+                result: Some(SecondCustomValue::new("xyz").into_value(Span::test_data())),
+            },
+            Example {
+                example: "custom-value generate2 { print }",
+                description: "Generate a new SecondCustomValue and pass it to a closure",
+                result: None,
+            },
+        ]
     }
 
     fn run(